This application claims the priority benefit of Canadian Application No. 2,304,337 filed on Apr. 7, 2000 as file no. 45082.1 and entitled Steelmaking Using Magnesium Carbonate.
The present invention relates to a method for creating a foaming slag in a primary steelmaking vessel.
Steel making is often a batch procedure involving several steps. Molten metal is continuously produced in a blast furnace to produce molten iron. The molten iron is transported in batches to a primary steelmaking vessel and is transformed into steel by blowing oxygen to remove carbon and phosphorus. The steel is usually formed in the presence of a primary slag which floats on the surface of the molten steel and is mostly comprised of oxides of calcium, silicon, iron, manganese, phosphorus, chromium and aluminum.
The primary steelmaking vessel may be an electric arc furnace which uses an electric arc to heat the molten iron or melt scrap steel to produce steel. It is desirable to have longer arcs to increase the temperature in the vessel, thereby decreasing the melting time of the process. However, longer arcs may result in increased arc instability or arc flaring. Arc instability or arc flare may cause consumption of the electrodes and cause hot gases and slag to be propelled against the refractory lining of the furnace, resulting in damage to or wear of the refractory lining.
It is well-known to reduce arc flare by producing a foaming slag in a primary arc furnace in an effort to bury the arc within the slag. For example, both U.S. Pat. Nos 4,447,265 and 4,528,035 describe procedures using the injection of carbon, lime, calcium dioxide and oxygen into a primary steelmaking vessel to produce a foaming slag. The carbon and oxygen react to form either carbon dioxide or monoxide which results in the foaming action. The lime is necessary to raise the slag viscosity because the foaming action will not occur if the slag viscosity is too low.
This process is expensive because of large amounts of relatively pure carbon and oxygen gas are required. The carbon source must be a relatively pure source of carbon such as metallurgical coke or anthracite coal. If the carbon source contains hydrogen or nitrogen impurities as is the case with petroleum coke or bituminous coal, the steel product may be compromised or yields may be reduced because hydrogen and nitrogen are soluble in steel at elevated temperatures. It would be an economic advantage to be able to reduce the amount and/or quality of the carbon source used to produce a foaming primary slag.
The addition of magnesium oxide (MgO) either as pure MgO or in the form of dolomitic lime to the primary slag is also well-known. Because the refractory lining material typically bears MgO, its addition to the primary slag may decrease refractory lining wear. The MgO added to the furnace is typically added very early in the heat because the equipment used to add the MgO is also used to add carbon as the process continues. This results in a higher than required MgO level early in the heat and may result in a MgO-deficient slag later in the process. While some MgO in the primary slag is desirable, too much MgO may be detrimental to subsequent desulphurization processes in secondary steel refining. It would be advantageous to be able to control MgO levels during the heat while controlling the formation of a foaming slag at the same time.
Furthermore, MgO is produced by calcining MgCO3 which releases CO2 directly into the atmosphere. As well, the heat necessary to calcine MgCO3 is obtained by burning fossil fuels. The release of CO2 is not desirable as it is a greenhouse gas which may cause global warming. It would be advantageous to eliminate the need to separately produce calcined MgO for use in the steelmaking process.
Therefore, there is a need in the art for a method and composition for producing a foaming slag in a primary electric arc furnace which mitigates the difficulties of the prior art.
The invention provides for methods and compositions for steelmaking using magnesium carbonate (MgCO3). In one aspect of the invention, the invention comprises a method of generating a foaming primary slag comprising the step of injecting an effective amount of a composition into a primary steelmaking furnace wherein said composition comprises at least about 30% MgCO3. The composition may further comprise a suitable carbon source. In one embodiment of the method, the composition used may comprise about 30% to about 100% MgCO3 and about 0% to about 70% carbon source. The carbon source may be chosen from chosen from the group comprising anthracite coal and metallurgical coke. In another embodiment of the method, the composition used may comprise about 65% to about 100% MgCO3 and about 0% to about 35% carbon source, in which case a lower grade of carbon source may be used. Alternatively, the composition may further comprise a small amount of CaO. in an amount preferably not exceeding about 20% of the composition.
In another aspect of the invention, the invention comprises a composition for injecting into a primary steelmaking furnace for generating a foaming primary slag, said composition comprising at least about 30% MgCO3. The composition may further comprise a suitable carbon source. In one embodiment, the composition may comprise about 30% to about 100% MgCO3 and about 0% to about 70% carbon. The carbon source may be chosen from the group comprising anthracite coal and metallurgical coke. In another embodiment of the composition, the composition may comprise about 65% to about 100% MgCO3 and about 0% to about 5% carbon source, in which case a lower grade of carbon source may be used. Alternatively, the composition may further comprise CaO, preferably in an amount not exceeding about 20%.